1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus provided with the same, and further specifically, relates to the sheet processing apparatus having a fold processing function and the image forming apparatus provided with the same.
2. Description of the Related Art
As an image forming apparatus such as a copying machine, a facsimile machine, and a printer, a publicly-known image forming apparatus has an apparatus body that forms an image on a sheet; a sheet stacking part that stacks the sheet ejected from inside to outside of the apparatus body, with the image formed thereon; and a detector that detects a near full state when the sheet is stacked in this sheet stacking part up to a state of almost a stacking limit amount (so-called a near full state) and stops an ejection of the sheet from the apparatus body. This detector is designed to detect the near full state in which a height of an uppermost surface of a bundle of sheets stacked in the sheet stacking part near the center (a thickness of the bundle of the sheets in a sheet plane near the center) reaches the height in the near full state.
When a sheet ejection is stopped, by taking out the bundle of sheets from the sheet stacking part by a user, the detector detects a state in which the height of the bundle of sheets on the sheet stacking part is reduced, and thereby, the sheet ejection is restarted.
In such an image forming apparatus, when a force from a carrying roller is added to the sheet passing through a bending part of a sheet carrying passage in the apparatus body, the sheet is curled in some cases. When the sheet having a curl is sequentially stacked in the sheet stacking part, usually a warp of a rear edge occurs rather than a central part of the sheet, thus increasing the height of the rear edge, thereby involving a problem that before detecting the near full state by the detector, the rear edge of the uppermost sheet comes above a sheet exit port of the apparatus body, thus allowing the sheet ejected next to collide with the rear edge of the bundle of sheets.
Therefore, instead of the aforementioned detector, the image forming apparatus having a curl detector that detects the rear edge of the sheet that warps due to curl, and stops the ejection of the sheet before the rear edge of the sheet comes above the sheet exit port, is proposed (for example, see Japanese Unexamined Patent Publication Nos. 9-309666 and 2006-82916).
In addition, the copying machine of recent years includes the one having a sheet processing apparatus that performs processing (so-called Z-shaped fold processing) for folding a sheet of A3 size into A4 size, on which an image is formed (for example, see Japanese Unexamined Patent Publication No. 2005-266245). This sheet processing apparatus is coupled to the apparatus body that forms the image on the sheet, and the sheet stacking part is disposed on a wall surface on the opposite side of the apparatus body, for arbitrarily performing the Z-shaped fold processing of the sheet fed from the apparatus body, with the image formed thereon, and ejecting this sheet.
In such a copying machine, image formation is started by selecting a “Z-shaped fold mode” in an operation part, and a Z-shaped folded sheet, with an image formed thereon, is ejected from a fold side and is sequentially stacked in the sheet stacking part. At this time, the Z-shaped folded sheet has a somewhat higher height on the fold side due to increase of thickness of a folded portion and a swelling of the fold. Note that when the Z-shaped folded sheet is stacked, the swelling of a lower sheet is slightly reduced by being compressed by a weight of upper sheets.
When the detector or the curl detector is applied to the sheet processing apparatus having the aforementioned Z-shaped folding function, as described above, the Z-shaped folded sheet has a higher height on the fold side. Therefore, the near full state is detected and the sheet ejection is stopped, even in a stage of smaller number of ejected sheets compared to a normal sheet, and even when there is almost no curl in the sheet. At this time, the height of the bundle of the Z-shaped folded sheets is gradually lowered due to reduction in swelling by its own weight of the sheets as described above, and therefore sheet ejection is stopped even when the sheet stacking part actually still has a room up to the sheet stacking limit amount. Here, the sheet stacking limit amount of the sheet stacking part means a sheet amount capable of preventing at least the sheet ejected from the sheet exit port, from colliding with the rear edge of the bundle of sheets.
In recent years, higher speed of image forming processing and post processing is increasingly desired, and therefore it is desirable to stack the Z-shaped folded sheets in the sheet stacking part as much as possible. However, it is undesirable to stop the sheet ejection in a state of such a small number of stacked sheets, because the number of times of suspending the processing is increased.